The useful life of A.C. series universal motors that typically find application in appliances, tools and the like, as well as in many industrial contexts, is usually limited to the length of the carbon brush, is rate of wear, and--in the case of replaceable brushes--the number of times the brushes can be replaced befor the commutator begins to wear severely.
It will be understood that the rate of wear of the brush is a function of the load, the speed of the motor, and most importantly, the spring pressure that is applied to the brush to keep it in bearing contact with the commutator. The curve of brush wear plotted against spring pressure, however, forms a parabola. Accordingly, it will be appreciated that with too much spring pressure the mechanical wear will become excessive, an improper film is formed on the commutator, and the brush life falls markedly. On the other hand, with too little pressure applied, the electrical arcing due to the high contact resistance and the mechanical abrasion due to brush bounce greatly reduce the possible life of the brush.
The typical motor brush rigging involves a helical spring bearing on the carbon brush, the two elements being combined in a box-like holder such that the brush is urged against the commutator. Although this design, as is understood, is used universally, it has the following limitations:
The pressure produced by a helical spring is a function of its compssion or extension. Therefore, when the brush assembly is brand new, and the brushes are at maximum length, the spring is at its fullest compression and the pressure therefore at its highest; at the end of the brush life, the spring extension is at its greatest and the pressure now is below the ideal. Therefore, depending on the spring rate, only a portion of the brush wear is in the ideal spring pressure range.
The long-life brush design uses a ribbon spring that is essentially wound like a clock spring and is set to unwind in such a direction as to hold the brush against the commutator. Since ribbon springs have an essentially constant force, the ideal pressure range can be obtained, thereby obtaining maximum wear on the brush from this aspect.
In addition to the force deflection curve, the helical spring also has a finite collapsed length. Accordingly, since the spring is generally located behind the brush in accordance with the usual way of enclosing it in the brush box, the space that it requires dictates that a shorter brush be used.
The long-life brush holders that have been designed and have now become conventional are such that the ribbon spring is arranged so that the coils providing the force are mounted outside of the brush holder on either side. Hence only a thin ribbon section of the spring is under the brush. Consequently, the long-life design has an additional advantage in that it provides extra space for a longer brush, and hence ensures the desired longer life.
Despite the advantages associated with the so-called long-life brush holders, a difficult problem arises with this design in that occasionally erratic brush life results due to the fact that the walls confining the spring coil portions tend to impede the unwinding or rotation of the coil portions. This is because of vibration and the back and forth motion of the brush. Also, the coil portions are riding back and forth, or in and out, as well as de-reeling in their receptacles.
It is well understood and appreciated that it is essential that the brush at all times follow the commutator. However, no matter how well one machines the commutator, the shaft and the bearing surfaces, there is bound to be some eccentricity to the system. Accordingly, it is very important to maintain not only the spring pressure, but the degree of freedon of the whole system to move.
It is therefore a primary object of the present invention to ensure that the proper spring pressure is applied to the brushes in a motor context at all times, and to guarantee complete freedom of movement for the system.
It is also understood that in the conventional design of the long-life brush holder, the springs, as they move back and forth in the receptacles, tend to hit the wall and drag on the bottom or floor of such receptacles. Additionally, debris such as carbon dust and the like is deposited in these areas, which further tends to reduce the freedom of the system to move.
It is therefore another primary object of the present invention to prevent the springs from moving back and forth in their receptacles such that they hit the wall or drag on the bottom or floor of such receptacles, it being a further object to cut down on the debris that tends to reduce the freedom of movement in the system.
The above and other objects are accomplished in accordance with first and second preferred embodiments of the present invention.